Effects of plasma spray parameters on TiO2-coated mild steel using design of experiment (DoE) approach

In this study, a design of experiment (DoE) method was utilised to identify the effect of air plasma spray (APS) parameters on several main properties of titanium dioxide (TiO2) coatings. Titanium dioxide (titania) feedstocks with sizes ranging from 10 μm to 45 μm were sprayed onto the mild steel substrates with different plasma spraying parameters. A 24 full factorial design was used to investigate the effects of four varying principal parameters at two levels, namely, the plasma power (20 and 40 KW), the powder feed rate (6 and 22 g/min), the scanning speed (0.2 and 0.5 m/s), and the number of cycles (10 and 20), on four important properties of coatings; microhardness, thickness/cycle, deposition efficiency (DE), and porosity. The results showed that one the most important factor in affecting all responses was the plasma power. It strongly affected the hardness and the porosity which had a primary effect on the thickness/cycle and the DE values. In contrast, the interaction of the powder feed rate and the scanning speed had a negative effects on both thickness/cycle and DE. The number of cycles has no profound effect on the considered responses; it can only be used as a factor to achieve different coating thicknesses. To conclude, a plasma power of 30 kW with a low level of powder feed rate of 6 g/min and 0.5 m/s of scanning speed is most preferable to optimise TiO2 coating deposition on mild steels.